The efficacy of behavioral weight loss treatment (BWL) has been demonstrated in large trials such as the Diabetes Prevention Program and LookAHEAD. However, the modality used for BWL delivery (frequent group treatment sessions over several months) has proven impractical outside the research setting. Thus, alternative methods of BWL delivery are need. Initial attempts to deliver BWL via the Internet and community organizations show promise and much has been learned about adapting BWL for delivery through non- traditional channels. However, the weight losses in these studies are often suboptimal due to poor behavioral adherence. We are therefore faced with the challenge of finding new methods of delivering empirically validated BWL that (1) are as efficacious as gold standard BWL, (2) facilitate behavioral adherence, and (3) show promise for dissemination if demonstrated efficacious. Our mobile health (mHealth) approach to delivering BWL via widely adopted smartphone technology shows promise for meeting this challenge. Smartphones are an ideal medium for delivering BWL because adherence may be improved by: (a) intervening on behaviors in real-time in the natural environment, (b) simplifying and personalizing self-monitoring, a vital weight loss strategy, (c) providing immediate personalized feedback to shape eating and physical activity behaviors, and (d) providing tailored education and skills training in frequent brief bursts. Furthermore, 44 percent of African Americans and Hispanics own a smartphone (30% of Whites) with >50% penetration expected this year. Given these benefits, there is a clear need to examine the efficacy of a smartphone-based BWL within a large-scale trial, and to identify strengths and weaknesses of this approach. We propose to randomly assign 270 overweight/obese adults to 18 months of (a) gold standard Group- BWL, (b) mHealth-BWL, or (c) Control. Participants in Group-BWL will attend group treatment sessions focused on diet, physical activity, and behavior modification. Groups will occur weekly, biweekly, and monthly during months 1-6, 7-12, and 13-18, respectively. Self-monitoring will be conducted via paper diaries with human feedback. Participants in mHealth-BWL will receive the same intervention components delivered via smartphone. Frequent, brief, videos will be used to deliver weight loss lessons. Self-monitoring will be conducted via apps with automatic and human feedback. An initial session and monthly weigh-ins will be used to facilitate adherence. The Control group will receive the same initial session and monthly weigh-ins, and human feedback on paper diaries to control for contact in mHealth-BWL. Assessments will be conducted at baseline, 3, 6, 12, and 18 months, with weight loss as the primary outcome. The proposed project addresses the significant problem of obesity by exploring the benefits of popular smartphone technology for delivering empirically validated behavioral strategies. If efficacious, an mHealth approach could be used to deliver BWL for relatively low cost using widely adopted smartphone technology.